Date: Mon, 3 Jul 2017 15:36:40 +0200 From: Ben RUBSON <ben.rubson@gmail.com> To: Freebsd fs <freebsd-fs@freebsd.org>, freebsd-scsi <freebsd-scsi@freebsd.org> Subject: Re: I/O to pool appears to be hung, panic ! Message-ID: <1F414ECE-1856-4EA3-A141-88B64703D4D6@gmail.com> In-Reply-To: <CAFLM3-qpsGx=EYHxAaLsSzF22JAJx0zg8deJ3FX_ec5uDO=0Cw@mail.gmail.com> References: <E8CC223E-3F41-4036-84A9-FBA693AC2CAA@gmail.com> <20170629144334.1e283570@fabiankeil.de> <A1CC7D73-4196-4503-9716-52E84AA24FD3@gmail.com> <C584B1DF-AC6E-4E77-9497-3D0EED76EACF@gmail.com> <CAFLM3-qpsGx=EYHxAaLsSzF22JAJx0zg8deJ3FX_ec5uDO=0Cw@mail.gmail.com>
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> On 03 Jul 2017, at 13:10, Edward Napierala <trasz@freebsd.org> wrote: >=20 > 2017-07-03 10:07 GMT+01:00 Ben RUBSON <ben.rubson@gmail.com = <mailto:ben.rubson@gmail.com>>: >=20 > > On 29 Jun 2017, at 15:36, Ben RUBSON <ben.rubson@gmail.com = <mailto:ben.rubson@gmail.com>> wrote: > > > >> On 29 Jun 2017, at 14:43, Fabian Keil <freebsd-listen@fabiankeil.de = <mailto:freebsd-listen@fabiankeil.de>> wrote: > > > > Thank you for your feedback Fabian. > > > >> Ben RUBSON <ben.rubson@gmail.com <mailto:ben.rubson@gmail.com>> = wrote: > >> > >>> One of my servers did a kernel panic last night, giving the = following message : > >>> panic: I/O to pool 'home' appears to be hung on vdev guid 122... = at '/dev/label/G23iscsi'. > >> [...] > >>> Here are some numbers regarding this disk, taken from the server = hosting the pool : > >>> (unfortunately not from the iscsi target server) > >>> https://s23.postimg.org/zd8jy9xaj/busydisk.png = <https://s23.postimg.org/zd8jy9xaj/busydisk.png>; > >>> > >>> We clearly see that suddendly, disk became 100% busy, meanwhile = CPU was almost idle. >=20 > We also clearly see that 5 minutes later (02:09) disk seems to be back = but became 100% busy again, > and that 16 minutes later (default vfs.zfs.deadman_synctime_ms), panic = occurred. >=20 > >>> No error message at all on both servers. > >> [...] > >>> The only log I have is the following stacktrace taken from the = server console : > >>> panic: I/O to pool 'home' appears to be hung on vdev guid 122... = at '/dev/label/G23iscsi'. > >>> cpuid =3D 0 > >>> KDB: stack backtrace: > >>> #0 0xffffffff80b240f7 at kdb_backtrace+0x67 > >>> #1 0xffffffff80ad9462 at vpanic+0x182 > >>> #2 0xffffffff80ad92d3 at panic+0x43 > >>> #3 0xffffffff82238fa7 at vdev_deadman+0x127 > >>> #4 0xffffffff82238ec0 at vdev_deadman+0x40 > >>> #5 0xffffffff82238ec0 at vdev_deadman+0x40 > >>> #6 0xffffffff8222d0a6 at spa_deadman+0x86 > >>> #7 0xffffffff80af32da at softclock_call_cc+0x18a > >>> #8 0xffffffff80af3854 at softclock+0x94 > >>> #9 0xffffffff80a9348f at intr_event_execute_handlers+0x20f > >>> #10 0xffffffff80a936f6 at ithread_loop+0xc6 > >>> #11 0xffffffff80a900d5 at fork_exit+0x85 > >>> #12 0xffffffff80f846fe at fork_trampoline+0xe > >>> Uptime: 92d2h47m6s > >>> > >>> I would have been pleased to make a dump available. > >>> However, despite my (correct ?) configuration, server did not dump = : > >>> (nevertheless, "sysctl debug.kdb.panic=3D1" make it to dump) > >>> # grep ^dump /boot/loader.conf /etc/rc.conf > >>> /boot/loader.conf:dumpdev=3D"/dev/mirror/swap" > >>> /etc/rc.conf:dumpdev=3D"AUTO" > >> > >> You may want to look at the NOTES section in gmirror(8). > > > > Yes, I should already be OK (prefer algorithm set). > > > >>> I use default kernel, with a rebuilt zfs module : > >>> # uname -v > >>> FreeBSD 11.0-RELEASE-p8 #0: Wed Feb 22 06:12:04 UTC 2017 = root@amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC > >>> > >>> I use the following iSCSI configuration, which disconnects the = disks "as soon as" they are unavailable : > >>> kern.iscsi.ping_timeout=3D5 > >>> kern.iscsi.fail_on_disconnection=3D1 > >>> kern.iscsi.iscsid_timeout=3D5 > >>> > >>> I then think disk was at least correctly reachable during these 20 = busy minutes. > >>> > >>> So, any idea why I could have faced this issue ? > >> > >> Is it possible that the system was under memory pressure? > > > > No I don't think it was : > > https://s1.postimg.org/uvsebpyyn/busydisk2.png = <https://s1.postimg.org/uvsebpyyn/busydisk2.png>; > > More than 2GB of available memory. > > Swap not used (624kB). > > ARC behaviour seems correct (anon increases because ZFS can't = actually write I think). > > Regarding the pool itself, it was receiving data at 6MB/s, sending = around 30kB blocks to disks. > > When disk went busy, throughput fell to some kB, with 128kB blocks. > > > >> geli's use of malloc() is known to cause deadlocks under memory = pressure: > >> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D209759 = <https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D209759>; > >> > >> Given that gmirror uses malloc() as well it probably has the same = issue. > > > > I don't use geli so I should not face this issue. > > > >>> I would have thought ZFS would have taken the busy device offline, = instead of raising a panic. > >>> Perhaps it is already possible to make ZFS behave like this ? > >> > >> There's a tunable for this: vfs.zfs.deadman_enabled. > >> If the panic is just a symptom of the deadlock it's unlikely > >> to help though. > > > > I think this tunable should have prevented the server from having = raised a panic : > > # sysctl -d vfs.zfs.deadman_enabled > > vfs.zfs.deadman_enabled: Kernel panic on stalled ZFS I/O > > # sysctl vfs.zfs.deadman_enabled > > vfs.zfs.deadman_enabled: 1 > > > > But not sure how it would have behaved then... > > (busy disk miraculously back to normal status, memory pressure due = to anon increasing...) >=20 > I then think it would be nice, once vfs.zfs.deadman_synctime_ms has = expired, > to be able to take the busy device offline instead of raising a panic. > Currently, disabling deadman will avoid the panic but will let the = device slowing down the pool. >=20 > I still did not found the root cause of this issue, not sure I will, > quite difficult actually with a stacktrace and some performance graphs = only :/ >=20 > What exactly is the disk doing when that happens? What does "gstat" = say? If the iSCSI > target is also FreeBSD, what does ctlstat say? As shown on this graph made with gstat numbers from initiator : https://s23.postimg.org/zd8jy9xaj/busydisk.png The disk is continuously writing 3 MBps before the issue happens. When it occurs, response time increases to around 30 seconds (100% = busy), and consequently disk throughput drops down to some kBps. CPU stays at an almost fully idle level. As shown here, no memory pressure : https://s1.postimg.org/uvsebpyyn/busydisk2.png = <https://s1.postimg.org/uvsebpyyn/busydisk2.png>; At the end of graphs' lines, panic is raised. iSCSI target is also FreeBSD, unfortunately ctlstat was not running = during the issue occurred. So numbers will be average since system startup (102 days ago). I also do not have gstat numbers from this disk on target side (to help finding if it's a hardware issue, a iSCSI issue or something = else). I will think about collecting these numbers if ever issue occurs again. > If everything else fails, you might want to do a packet trace using = "tcpdump -w" and take > a look at it using Wireshark; it decodes iSCSI quite nicely.
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